The present invention relates to an air electrode, more specifically to an air electrode suitable for use in a hydrogen/oxygen fuel cell, a metal/air cell or an oxygen sensor.
There have hitherto been used gas diffusion electrodes for air electrodes such as various fuel cells, air-metal cells typically including air/zinc cells, and Galvanic oxygen sensors. In the initial period, thick and uniformly porous electrodes were used as the gas diffusion electrodes, in many cases. However, in order to satisfy the requirements for thinness and leakage-proofing, it has recently become common to use a dual electrode prepared by integrating an electrode body and a hydrophobic layer and adapted to carry out an electrochemical oxygen reduction reaction (Japanese Patent Publication No. 25684/1968).
Namely, in such air electrodes, it has been common to use as the hydrophobic layer, a fluorine-containing resin such as a polytetrafluoroethylene, a polytetrafluoroethylene-hexafluoropropylene copolymer, or a polyethylene-tetrafluoroethylene copolymer, or polypropylene, in a form of a porous material including, for instance, a sintered powder material having a particle size of from 0.2 to 40.mu., a paper-like non-woven fabric material prepared by heat treatment of fibers, a similar woven fabric material, a powder material partially replaced by a fluorinated graphite, a film material prepared by rolling fine powder together with a pore-increasing agent or a lubricant oil, followed by heat treatment, or a film material prepared by rolling without being followed by heat treatment (Japanese Patent Publication No. 44978/1973). Further, in a case where no fluid leakage is allowed, for instance, in the case of an air electrode for Galvanic oxygen sensor to be used for detecting the concentration of oxygen gas dissolved in water, a thin gas-permeable non-porous film resistant to an electrolyte has been used on the gas side. An air electrode used has been constructed by integrating such a water repellent layer or gas permeable film and a porous electrode as the electrode body by pressing or by means of an adhesion, or by coating such a water repellent layer with an electrode body-forming material (Battery Handbook, Denki Shoin, P. 2-135).
The electrode body in this case is formed by integrating active carbon powder carrying a catalyst such as nickel tungstate having a low oxygen reduction over-voltage, tungsten carbide coated with palladium-cobalt, nickel, silver, platinum or palladium, with a porous metal body, a porous carbon body or a non-woven carbon fiber fabric, with use of a binder such as polytetrafluoroethylene.
However, there still remain some problems with the conventional air electrode, e.g., a thin air electrode for an air/zinc cell where it is required to be thin, completely free from fluid leakage and useful for heavy duty discharge.
For instance, in the case where a porous body prepared by sintering a fluorine-containing resin powder is used as the hydrophobic layer, continuous discharge under fairly heavy duty at a level of about 20 mA/cm.sup.2 can be done, but the thickness is required to be at a level of from 0.125 to 0.50 mm, and since the pore sizes are not uniform and there exist pores of large diameters, it is likely that due to, e.g., the volume expansion at the opposite electrode to the air electrode, the inner pressure of the cell increases, thus leading to fluid leakage, especially in the case of a sealed type. On the other hand, in an air electrode wherein a thin gas permeable non-porous film is provided at the gas side, e.g., by means of an adhesive or the like, to prevent fluid leakage, it is possible to completely prevent fluid leakage, and to make the thickness as thin as about 12.5 .mu.m. In this case, however, it would become highly difficult to carry out continuous discharge at a large current at a level of at least 10 mA/cm.sup.2.
Further, there has been known a so-called teflon-bonded air electrode in which carbon or nickel powder is used as the major component and PTFE (polytetrafluoroethylene) powder is dispersed therein. However, in such an electrode, a hydrophilic surface is exposed to a substantial extent and an electrolyte tends to gradually penetrate into the electrode through the surface, whereupon no sufficient diffusion of the gas into the electrode will be done. Thus, it has a drawback that the stability of the heavy duty characteristic of the electrode is thereby impaired.
It is conceivable that this is caused as follows: the PTFE used as the binder is hardly soluble in a solvent such as water, and it is used in a form of powder or a dispersion. However, the minimum size of the PTFE particles in the dispersion is at a level of about 0.2 .mu.m, and it is difficult to obtain a dispersion of particles having smaller size. Accordingly, unless the size of pores in the active carbon or porous sintered material is sufficiently large relative to the particle size of the PTFE, penetration of the PTFE particles into the pores can not be expected. Thus, a hydrophilic surface would remain in the electrode. It has been proposed to enlarge the size of pores of the porous sintered material to a level greater than the size of the dispersed PTFE particles so as to permit the PTFE particles to penetrate deeply into the pores.
However, if the size of the pores of the electrode is so enlarged, the structure of the three phase interface effective for discharge reaction becomes coarse, and the surface area is decreased, whereupon it becomes impossible to obtain a large current. Besides, no penetration into fine pores such as pores of the active carbon is expected. Thus, there has not yet been found one which exhibits adequate characteristics for practical purposes.